Microelectronic device assemblies, such as memory devices and microprocessors, typically include one or more microelectronic components attached to a substrate. The microelectronic components commonly include at least one die having functional features such as memory cells, integrated circuits, and interconnecting circuitry. The dies of the microelectronic components may be encased in a plastic, ceramic, or metal protective covering. Each die commonly includes an array of very small bond-pads electrically coupled to the functional features. The bond-pads can be used to operatively connect the microelectronic component to the substrate.
One type of microelectronic component is a “flip-chip” semiconductor device. These components are referred to as “flip-chips” because they are typically manufactured on a wafer and have an active side with bond-pads that initially face upward. After manufacture is completed and a die is singulated, the die is inverted or “flipped” such that the active side bearing the bond-pads faces downward for attachment to a substrate. The bond-pads are usually coupled to terminals, such as conductive “bumps,” that electrically and mechanically connect the die to the substrate. The bumps on the flip-chip can be formed from solders, conductive polymers, or other materials. In applications using solder bumps, the solder bumps are reflowed to form a solder joint between the flip-chip component and the substrate. This leaves a small gap between the flip-chip and the substrate. To enhance the integrity of the joint between the microelectronic component and the substrate, an underfill material is introduced into the gap. The underfill material bears some of the stress placed on the components and protects the components from moisture, chemicals and other contaminants. The underfill material can include filler particles to increase the rigidity of the material and modify the coefficient of thermal expansion of the material.
The underfill material typically is dispensed into the underfill gap by depositing a bead of the underfill material along one or two sides of the flip-chip when the underfill material is in a fluidic state (i.e., flowable). As shown schematically in FIG. 1, a bead of an underfill material U may be dispensed along one side of the die D. The flowable underfill material will then be drawn into the gap between the die D and the substrate S by capillary action. The direction of this movement is indicated by the arrows in FIG. 1. After the underfill material fills the gap, it is cured to a hardened state. Although such a “single stroke” process yields good results, the processing time necessary to permit the underfill material U to flow across the entire width of the die can reduce the throughput of the manufacturing process.
FIG. 2 illustrates an alternative approach wherein the underfill material U is applied in an L-shaped bead along two adjacent sides of the die D. By reducing the average distance that the underfill material has to flow to fill the underfill gap, processing times can be reduced. The L-stroke approach, however, can lead to more voids in the underfill material, which adversely affect the integrity of the bond between the die D and the substrate S.
In the single stroke and L-stroke approaches, the filler particles can become segregated from the polymer fluid as the underfill material flows across the die. Consequently, one side of the flip-chip often has a greater concentration of filler particles. The nonuniform distribution of filler particles creates differences in the rigidity and the coefficient of thermal expansion of the underfill material across the die.
In other embodiments, the underfill material may be deposited across a plurality of dies at the wafer-level to form an underfill layer. After the underfill layer is formed, the dies can be singulated and attached to substrates. Forming an underfill layer with filler particles on a die before attaching a substrate to the die has some drawbacks. For example, the filler particles in the portion of the underfill layer above the conductive bumps can obstruct the connection between the conductive bumps of the die and the substrate. To prevent the filler particles from interfering with the connection, one approach is to form two underfill layers on the die. The first underfill layer includes filler particles and has a thickness no greater than the height of the conductive bumps. The second underfill layer is formed over the first layer and does not contain filler particles. This approach, however, requires two dispensers and two types of underfill material. Another approach is to form the underfill layer on the die at the wafer-level before forming the conductive bumps. Next, vias are formed in the underfill layer and the conductive bumps are formed in the vias. This approach, however, is complicated and can result in contamination of the underfill layer and/or the conductive bumps. Moreover, it is difficult to deposit solder paste in very small vias. Another approach is to form the underfill layer over the die and the conductive bumps, then remove the top portion of the underfill layer so that the underfill layer has a thickness equal to the height of the conductive bumps. This approach also is complicated, requires cleaning, and may contaminate the device. Accordingly, a new method for forming an underfill layer that has filler particles is needed.